1. Field of the Invention
This invention relates to the field of electro-optical modulator driver circuits, and particularly to modulator driver circuits requiring a high output voltage swing and high speed.
2. Description of the Related Art
Fiber optic cables are often driven by means of a continuous laser modulated by an electro-optical modulator. One such modulator that is often used in high-bit-rate (˜10 Gb/s or more) communication systems is the Mach-Zehnder (MZ) modulator. MZ modulators use an optical interferometer fabricated in nonlinear material—e.g. Lithium Niobate, in which the velocity of light can be varied by an applied, electric field—to either block or transmit laser light as a function of an externally applied driving voltage. This voltage is either applied to a single input (“single-ended” drive) or is applied differentially at two inputs (in a “dual-drive” MZ modulator). The dual-drive modulator uses less voltage per input, but its two input signals require precise amplitude and phase matching which adds to system complexity. The single-ended MZ modulator is therefore more widely used today.
Due to the weakness of electro-optic effects, MZ modulators require a substantial driving voltage (“V pi”)—typically 6–8 volts p—p for single-ended drive—to create high-quality light pulses at data rates of 10 Gb/s or more. However, data signals produced by digital circuits such as multiplexers are of relatively small amplitude, e.g. 200 millivolts. Thus it is necessary to provide a modulator driver that amplifies the data signal up to the required voltage. Since the modulator operates at the full serial data rate of the optical channel, the output of the modulator driver usually has the largest amplitude and highest bandwidth of any electrical signal in the system. The modulator also transfers any variation or imperfection in the amplified electrical waveform directly to the optical signal, and this distortion significantly affects the bit-error rate and thus the usable distance of the fiber optic link. These factors make the circuit design of the modulator driver especially critical.
The fundamental problem in MZ modulator driver design is obtaining high bandwidth (fast rise and fall times) with high pulse fidelity (damped response and low jitter) while using transistors large enough to handle the high voltage and current required to drive typical 50-ohm MZ modulators. At the present time, most MZ modulator drivers for bit rates of 10 Gb/s or more employ a distributed amplification principle (also known as a traveling wave amplifier—TWA); such an amplifier is shown in FIG. 1a. Here, the large transistor required to provide the high-amplitude output is split into a number of smaller transistor cells 10, which are interconnected using inductors or transmission lines 12. The smaller cells break up the input and output capacitances of the transistor, so that the inductors can quickly charge and discharge them in sequence (similar to a traveling wave in a transmission line) rather than all at once. This increases the circuit's bandwidth, but also greatly expands its physical size and results in limited integrability with other circuits. Moreover, TWA-based drivers have low electrical gain since they basically comprise a single (albeit large) transistor. Several cascaded TWA chips are thus required to provide enough amplification to form a practical driver, and each chip requires its own external components such as bias tees and dc-blocking/decoupling capacitors.
TWA-based drivers are predominantly built in a III–V field effect transistor (FET) technology such as gallium arsenide (GaAs) PHEMTs due to their combination of high speed and high breakdown voltage. The TWA architecture is also best suited to a device with purely capacitive input characteristics like an FET gate. The disadvantage of PHEMT devices is that their varying threshold voltage and conductance characteristics make them inappropriate for creating either high-precision analog or low-power digital control circuits. The result is that PHEMT-based modulator drivers generally require external (off-chip) control circuits to adjust their operation and keep their output stable over normal operating conditions like fluctuating temperature and supply voltage. These external circuits add considerable cost and complexity to the system.
For higher precision other driver architectures may be employed, such as the differential transistor pair with current source shown in FIG. 1b. In this approach, the large output transistors are “lumped” in one place rather than distributed as in the TWA. This has the advantage that the output amplitude may be precisely controlled through the use of a regulated current source. Bipolar transistors such as HBT devices are well suited to this configuration due to their high transconductance, uniform and well-modeled electrical characteristics, and compact size. To be suitable for MZ modulator applications, the transistors must be large enough to deliver current exceeding 100 milliamperes into load impedances of approximately 25 to 50 ohms while handling the resulting voltage swing, 6 volts p—p for example. Both pair transistors 14, 16 of this size will have significant parasitic capacitances (Cp) between their input and output. These capacitances, multiplied by the Miller effect that arises due to the inverting voltage gain in this common-emitter configuration, tend to reduce the stage's bandwidth and cause unacceptably slow rise time. This capacitance also degrades the output return loss of the driver, which leads to jitter when operating with an impedance-mismatched load. Accordingly, no examples have been found in the prior art of a high-speed single-ended MZ modulator driver (more than 6 volts p—p at 10 Gb/s or higher bit rate) using a lumped bipolar transistor output stage.
Some “lumped” amplifier architectures attempt to mitigate the problems associated with the use of larger transistors. One such approach is shown in FIG. 1c. Here, differential pair transistors 18 and 20 are cascoded with transistors 22 and 24, respectively. This lowers the impedance looking into the cascode transistors' emitters, reducing the Miller effect on the pair transistors and thereby improving the circuit's speed. However, this approach requires the use of four large devices, which can make the circuit layout difficult. In addition, the stacked transistors require a higher supply voltage, and thus the circuit consumes more DC power. Finally, the circuit's differential inputs require a differential pre-driver, which again requires more power.